Back2. Mendelian Genetics
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2. Mendelian Genetics
Gregor Mendel and the Discovery of Genetic Transmission
Gregor Mendel, often referred to as the father of genetics, discovered the basic principles of genetic transmission through his experiments with pea plants (Pisum sativum). His work laid the foundation for modern genetics by demonstrating how traits are inherited from one generation to the next.
Background: Mendel pursued natural sciences and chose to study heredity in peas, which offered distinct, easily observable traits.
Experimental Approach: He obtained 34 varieties of peas and identified 14 strains representing seven specific traits, each with two distinguishable forms.
Significance: Mendel's findings, published in 1866, were not appreciated until their rediscovery in 1900, which revolutionized biology.
Mendel’s Experimental Innovations
Mendel’s success was due to his rigorous scientific method and several critical experimental innovations.
Controlled Crosses: Pea plants can self-fertilize or be cross-fertilized. Mendel used artificial cross-fertilization by removing anthers and introducing pollen with a brush.
Pure-Breeding Strains: He established strains that consistently produced the same phenotype, starting each experiment with pure-breeding parental (P) generation plants.
Selection of Dichotomous Traits: Mendel selected traits with only two possible phenotypes, such as yellow vs. green seed color.
Quantification of Results: He counted large numbers of progeny and identified consistent ratios between phenotypes.
Replicate, Reciprocal, and Test Crosses: Mendel repeated crosses, reversed parental sexes (reciprocal), and performed test crosses to determine unknown genotypes.
The Seven Dichotomous Traits of Pisum sativum
Mendel studied seven traits, each with a dominant and recessive phenotype.
Trait | Dominant Phenotype | Recessive Phenotype |
|---|---|---|
Seed color | Yellow | Green |
Seed shape | Round | Wrinkled |
Pod color | Green | Yellow |
Pod shape | Inflated | Constricted |
Flower color | Purple | White |
Flower position | Axial | Terminal |
Plant height | Tall (72–84 cm) | Short (18–24 cm) |
The Blending Theory of Inheritance
Before Mendel, the blending theory suggested that offspring traits were a mix of parental traits, with no reappearance of original traits in subsequent generations. Mendel’s experiments disproved this theory.
Example: Under blending theory, crossing a black and white cat would yield only gray kittens, with no black or white traits reappearing.
Mendel’s Scientific Method
Mendel’s approach followed the modern scientific method:
Make initial observations
Formulate a testable hypothesis
Design a controlled experiment
Collect data
Interpret results
Draw conclusions and reformulate hypothesis if necessary
Key Point: Careful experimental design was central to Mendel’s success.
Monohybrid Crosses and the Segregation of Alleles
Monohybrid crosses involve organisms heterozygous for one gene. Mendel’s experiments revealed the segregation of alleles and the concept of dominant and recessive traits.
Dominant Phenotype: The trait expressed in the F1 generation (e.g., yellow peas).
Recessive Phenotype: The trait not expressed in F1 but reappearing in F2 (e.g., green peas).
Homozygous: Individuals with identical alleles for a trait.
Heterozygous: Individuals with different alleles for a trait.
Consistent Experimental Results
Dominance of one phenotype in F1
Reemergence of recessive phenotype in F2
Approximate 3:1 ratio of dominant:recessive in F2
Table: Mendel’s Observations for Seven Monohybrid Traits
Cross | F1 Dominant | F1 Recessive | F2 Ratio |
|---|---|---|---|
Round x wrinkled seed | 5474 round | 1850 wrinkled | 2.96:1 |
Yellow x green seed | 6022 yellow | 2001 green | 3.01:1 |
Purple x white flower | 705 purple | 224 white | 3.15:1 |
Inflated x constricted pod | 882 inflated | 299 constricted | 2.95:1 |
Green x yellow pod | 428 green | 152 yellow | 2.82:1 |
Axial x terminal flower | 651 axial | 207 terminal | 3.14:1 |
Tall x short plant | 787 tall | 277 short | 2.84:1 |
Particulate Inheritance and Alleles
Mendel proposed the theory of particulate inheritance, stating that organisms carry two discrete hereditary units (alleles) for each trait, one from each parent. These alleles determine the phenotype.
Alleles: Hereditary particles represented by letters (e.g., G/g for seed color).
Monohybrid Cross Ratios
Phenotypic Ratio: 3:1 (dominant:recessive) in F2
Genotypic Ratio: 1:2:1 (homozygous dominant : heterozygous : homozygous recessive)
Example Punnett Square for G/g x G/g:
Gametes: G and g from each parent
Offspring: GG, Gg, Gg, gg
Mendel’s First Law: Law of Segregation
The law of segregation states that the two alleles for each trait separate during gamete formation, and randomly unite at fertilization. This law applies to all seven traits Mendel studied.
Equation: , ,
Test-Cross Analysis
Test crosses are used to determine the genotype of an organism with a dominant phenotype by crossing it with a homozygous recessive individual.
If the dominant individual is heterozygous, offspring will be 1:1 dominant:recessive.
If homozygous, all offspring will show the dominant trait.
Test Cross | Dominant Progeny | Recessive Progeny | Ratio |
|---|---|---|---|
Round seed x wrinkled seed | 193 round | 192 wrinkled | 1.01:1 |
Yellow seed x green seed | 196 yellow | 189 green | 1.04:1 |
Purple flower x white flower | 85 purple | 81 white | 1.05:1 |
Tall plant x short plant | 87 tall | 79 short | 1.10:1 |
Key Terms and Definitions
Phenotype: Observable trait or characteristic.
Genotype: Genetic makeup of an organism (combination of alleles).
Allele: Alternative form of a gene.
Homozygous: Having two identical alleles for a trait.
Heterozygous: Having two different alleles for a trait.
P Generation: Parental generation in a genetic cross.
F1 Generation: First filial generation, offspring of the P generation.
F2 Generation: Second filial generation, offspring of F1 individuals.
Example Application
If a pea plant with yellow seeds (dominant) is crossed with a plant with green seeds (recessive), and all F1 offspring are yellow, the yellow parent is likely homozygous. If F1 plants are self-fertilized, the F2 generation will show a 3:1 ratio of yellow to green seeds.
Additional info: These notes expand on the original slides by providing definitions, context, and examples for key genetic concepts, ensuring a comprehensive understanding suitable for college-level genetics students.
